The present invention relates to a method and apparatus for displaying an optically corrected image using a small throw ratio, off axis projection display system. In a is particular embodiment, the present invention relates to an off-axis integrated front projection system having a throw ratio less or equal to 1.0 that coordinates specialized projection optics and electronics optimized to work together to create a high-quality viewing image and correct lens induced optical distortion, keystone distortion, and anamorphic distortion.
Electronic display systems are devices capable of presenting electronically generated images. Whether for use in home-entertainment, advertising, videoconferencing, computing, data-conferencing or group presentations, the demand exists for an appropriate display device.
Image quality remains a very important factor in choosing a video display device. However, as the need increases for display devices offering a larger picture, factors such as cost and device size and weight become vital considerations. Larger display systems are preferable for group or interactive presentations. The size of the display system cabinet has proven an important factor, particularly for home or office use, where space to place a large housing or cabinet may not be available.
Currently, the most common video display device is the typical CRT monitor, usually recognized as a television screen. CRT devices are relatively inexpensive for applications requiring small to medium size images (e.g., 9xe2x80x3 to 27xe2x80x3, xcx9c23 to 70 cms). (image size traditionally is measured along the diagonal dimension of a rectangular screen). However, as image size increases, the massive proportions and weight of large CRT monitors become cumbersome and severely restrict the use and placement of the monitors. Also, screen curvature issues appear as the screen size increases. Large CRT monitors consume a substantial amount of electrical power and produce significant electromagnetic radiation. Finally, the cost of very large CRT monitors may be prohibitive for many applications.
A new category of presentation systems includes so-called thin plasma displays. Much attention has been given to the ability of plasma displays to provide a relatively thin (about 75-100 mm) cabinet, which may be placed on a wall as a picture display in an integrated compact package. However, at the present time, plasma displays are costly and suffer from the disadvantages of low brightness (approx. 200-400 cd/m2 range) and difficulty in making repairs. Plasma display panels are heavy (xcx9c80-100 lbs., 36-45 kg.), and walls on which they are placed may require structural strengthening.
A traditional type of video presentation device is the projection system, including both rear and front projection. In projection systems, one or more imagers creates an image that is projected using optical lenses. An imager generally is an electronically controlled array of pixels that can be turned on or off to create an image. Imagers, or light valves as they are sometimes called, may be reflective (an xe2x80x9conxe2x80x9d pixel reflects incident light to form the image) or transmissive (an xe2x80x9conxe2x80x9d pixel transmits incident light). Common imager types include liquid crystal display devices and digital micromirror devices.
Rear projection generally comprises a projection mechanism or engine contained within a large housing for projection to the rear of a transmissive screen. Back-projection screens are designed so that the projection mechanism and the viewer are on opposite sides of the screen. The screen has light transmitting properties to direct the transmitted image to the viewer.
A front-projection system is one that has the projection mechanism and the viewer on the same side of the screen. Front projection systems present many different optical and arrangement challenges not present in rear projection systems, as the image is reflected back to the audience, rather than transmitted. An example of a front projection system is a portable front projector and a front projection screen, for use in meeting room settings or in locations such as an airplane cabin.
Front projection systems have traditionally not been considered attractive for interactive applications because of factors such as blocking of the image by the projector or the presenter and image distortion.
Traditional electronic front projectors typically require a room that affords the projection volume necessary for image expansion without physical obstructions. Although images may be projected upon a large flat surface, such as a wall, better image quality is achieved by the use of a separate screen. FIGS. 1 and 2 illustrate a traditional front projection system. A projector 10 is placed on a table or other elevated surface to project an image upon a screen or projection surface 20.
Traditional integrated projectors require optical adjustment, such as focusing every time the projector is repositioned, as well as mechanical adjustment, such as raising of front support feet to position the image on the projection screen. Electronic connections, such as those to a laptop computer, generally are made directly to the projector, thus necessitating that the projector be readily accessible to the presenter or that the presenter runs the necessary wiring in advance.
To achieve a suitable image size, and also due to focus limitations, the projector 10 requires a certain xe2x80x9cprojection zonexe2x80x9d and distance from the screen 20. Table A lists the published specifications for some common electronic projectors currently in the market.
Throw distance is defined as the distance from the projection lens to the projection screen measured along the optical axis of the projection lens. Throw ratio usually is defined as the ratio of throw distance to screen diagonal. Short throw distance is defined as at most one meter. To achieve a large image, between xcx9c40 to xcx9c60 inches (xcx9c1 to xcx9c1.5 meters), a projector having a throw ratio of approximately 1.5 must be positioned at least 3.5 to 7.5 feet (approximately xcx9c1.5 to xcx9c2.25 meters) away from the wall or screen.
The existence of this xe2x80x9cprojection zonexe2x80x9d in front of the screen prevents the viewer from interacting closely with the projected image. If the presenter, for example, wishes to approach the image, the presenter will block the projection and cast a shadow on the screen.
The projection zone may be reduced by moving the projector closer or off-axis from the screen. However, optical distortion effects significantly affect the quality of a projected image at short throw, small throw ratio, and offset angles. There are three distortion effects of particular concern in off-axis projection, especially in front projection systems: keystone geometric distortion, anamorphic geometric distortion, and projection lens imaging distortions, such as third order pincushion distortion. The effects of these distortion components are increased, the closer the projection lens is to the screen.
Those familiar with the use of electronic projectors will appreciate that placing the projector at an angle to the central normal axis of the screen (i.e., off-axis) produces a trapezoidal shape distortion of the image, known as a keystone effect. Keystoning is a geometric image distortion where the projection of a rectangular or square image results in a screen image that resembles a keystone or trapezoid, that is a quadrilateral having parallel upper and lower sides, but said sides being of different lengths.
Methods for the reduction of keystoning again are dependent upon the position of the projector with respect to the screen. A measure of keystone correction may be achieved by optical and by electronic methods. For moderate (10xc2x0-20xc2x0 off axis) keystone correction in LCD projectors, optical methods are presently preferable, as electronic methods may suffer from pixelation, as pixels become misaligned with the image features. Most new electronic projectors offer a limited degree of optical keystone correction, often achieved by mechanical offset of the projection components. However, the placement of the projector at moderate offset angles may still interfere with the line of sight of the audience.
Anamorphic distortion causes a projected image to be stretched unequally in the vertical direction above and below the optical axis centerline.
Pincushion distortion is a third order distortion generated by the projection lens. The degree of pincushion distortion is related to the complexity of the lens and to the lens design.
Available optical keystone correction in presently commercially available large throw ratio and throw distance portable electronic front projectors generally ranges between 10xc2x0 to 20xc2x0. While it may be theoretically possible to correct these distortion components in small throw-ratio, off-axis systems by optical means, the cost of developing and manufacturing specialized exotic lenses may be so high as to destroy the commercial attractiveness of such a system. Similarly, electronic means of image distortion correction are deficient at short throw (less than 1 meter), small throw ratio (less or equal to 1), off axis projection, due to the effects of image pixelation and misalignment.
Image pixelation occurs when the pixels in a projection imager become misaligned with the image features. No commercially available correction mechanism has addressed satisfactorily the issues of optical distortion and image quality in small throw-ratio, short throw, off-axis front projection systems.
A newer means for electronic image correction is discussed in U.S. Pat. No. 5,594,676, issued to Greggain, et al., entitled xe2x80x9cDigital Image Warping Systemxe2x80x9d and in J. Goel, et al., xe2x80x9cCorrecting Distortions in Digital Displays and Projectors Using Real-Time Digital Image Warping,xe2x80x9d SID 99 Digest, pp. 238-241 (hereinafter xe2x80x9cSID Articlexe2x80x9d). Under this system of electronic correction an image handling microchip predistorts the output image on an imager (such as an LCD or DMD imager) to compensate for distortions introduced by the rest of the system. This system corrects over a wider range and corrects more types of distortion.
However, such system requires extensive computing power and a dedicated image handling IC. Even more importantly, the pre-distortion of the image causes significant loss of data. For example, an image in an XGA imager contains 1024xc3x97768 pixels of image data. If the image is pre-distorted as discussed in the SID Article, only a fraction of those pixels may be used. The imager bounds the entire pre-distorted image. Therefore, in the best case scenario, if the pre-distorted image may be correctly formatted to use the maximum possible area in the imager, only the widest point of the pre-distorted image would use all of the pixels in a row. Similarly, only the tallest portion of the pre-distorted image would use all of the pixels in a column. For all other rows and columns, pixel data necessarily will have to be omitted. As the amount of distortion increases, the shape variations of the pre-distorted image increase. Therefore, the more severe the correction necessary, the more information will be lost. While such information loss may not be critical in some applications, the data loss may severely affect certain applications, such as static presentations including small features or numbers.
The need remains for a large screen video presentation system that offers efficient space utilization, and allows for user interaction. Such a system should preferably correct the various distortion components within a displayed image while minimizing data loss.
The present invention provides a method for manufacture for an off-axis, small throw ratio projection system. The system of the present invention greatly reduces the projection zone by placing the projector at a short distance and off-axis from the projection screen. A reduced and displaced projection zone offers immediate and attractive advantages. A large image achieved by a projector closer to the screen allows the presenter to get closer to the screen without interfering with the image and reduces the space required for front projection.
However, small throw ratio (where the throw ratio is less equal to 1), off-axis projection induces large distortion effects. The present invention addresses these distortion effects by a novel method of combining optical correction with electronic correction.
The present invention provides a projection system and associated method that corrects, that is reduces or eliminates, distortion components within a projected image. In one embodiment, the present invention includes a front projection system having a front projector device coupled to a projection screen. The front projector device is characterized by off-axis projection and a throw ratio of at most 1, which generate distortion including pincushion, keystone, and anamorphic distortion components. The projection device has an electronic distortion correction component operable to pre-distort an image prior to projection. The method of the present invention combines optical and electronic correction in order to correct for the pincushion, keystone and anamorphic distortion components of the projector device.
In one embodiment, the optics includes a 9.44 mm focal length wide-angle projection lens that generates approximately 10% pincushion distortion. The electronic distortion correction component includes an integrated circuit chip having image pre-distortion functions. Off-axis optics further generate approximately 74% keystone distortion and approximately 34% anamorphic distortion. According to another aspect of the present invention, a method is disclosed for correcting distortion generated in a projection system by a combination of optical and electronic correction. The method includes setting a limit on the amount of image information that is acceptable to lose through the optical components, and selecting an optical solution, including optics having inherent distortion within the set limit. An electronic correction component is then selected that is operable to pre-distort an image to correct for remaining distortion not corrected by the optics of the optical solution. In a particular embodiment, the projection lenses are placed at a predetermined distance and angle from the projection screen. Electronic correction is achieved by xe2x80x9cwarpingxe2x80x9d the electronic image in accordance with a warp map that accounts for the particular distortion components at the specific throw distance and projection angle.